The present invention relates to the field of semiconductor processing and, more particularly, to optoelectronic devices.
A continuing object of integrated circuit manufacturing is to increase the speed of operation. One obstacle is the speed at which signals for the source of information are sent to the integrated circuit. Optical devices are being investigated to increase the speed of operation by using light as opposed to an electrical signal as the source.
Since silicon substrates are typically used to form integrated circuits, it is desirable to build the optical devices with integrated circuitry on silicon substrates. However, the physical properties of silicon, such as its low absorption coefficient compared to germanium, make optoelectronic devices in silicon inefficient. One way to increase the efficiency is to form a resonant cavity detector within a silicon substrate. A resonant cavity detector includes, two mirrors or distributed Bragg reflectors (DBR) vertically separated from each other by a silicon layer. When light is applied to the resonant cavity device the mirrors bounce the light between each other and multiply the light intensity. The greater the light intensity the more number of electron and hole pairs will be formed in the silicon to transmit a signal.
If the silicon layer between the two mirrors is polysilicon, the current flowing in the absence of radiation (dark current) is large, which decreases the efficiency of the device. However, monocrystalline silicon has a low dark current and is therefore used as the silicon layer in resonant cavity devices.
One way the prior art forms a resonant cavity device with a monocrystalline silicon layer between the mirrors is to form the bottom mirror, etch two holes within the bottom mirror to expose the underlying silicon substrate and epitaxially grow silicon through the two holes and laterally across the top surface of the bottom mirror. Afterwards, the upper mirror is formed within the epitaxially grown silicon. By growing the semiconductor material laterally across the top surface of the bottom mirror the semiconductor material also grows vertically to a thickness much greater than desired. To reduce the thickness of the epitaxial layer, a chemical mechanical polish (CMP) or etch back step is needed. This process is undesirable for manufacturability because the epitaxial growth process is slow and the CMP or etch back step increases cycle time by adding additional steps. Additionally, the epitaxial growth introduces defects in the epitaxially grown silicon at the locations where the lateral overgrowth of the silicon meets over the top surface of the mirror. Thus, a more manufacturable process to form a monocrystalline semiconductor material between two mirrors in an optical device is needed.